Manufacture of hydroxylated ferric sulphate compound

ABSTRACT

A process for the manufacture of ferric sulphate from ferrous sulphate in a closed circuit or vessel comprising a liquid phase and a vapor phase, the process comprising the oxidation between about 70° C. to about 150° C. of Fe ++  →Fe +++  under pressure utilizing oxygen in the closed circuit or vessel using (NO x ) as a catalyst where x is a number between 1 and 2 inclusive of 1 and 2, {NO x  being a mixture of NO and NO 2  } and wherein the process comprises the following reactions: 
     1) NO+1/2O  2  →NO 2 , and 
     2) 2FeSO 4  +H 2  SO 4  +NO 2  →Fe 2  (SO 4 ) 3  +H 2  O+NO, and wherein the oxidation of Fe ++   is affected by spraying the reacting solution (taken for example from the bottom of the vessel or closed circuit containing the reacting solution) through a reacting cloud in the vapor phase comprising NO, NO 2  and O 2  enclosed in the vapor space of the closed circuit of vessel. 
     The process of manufacture is also applicable for the manufacture of hydroxylated ferric sulphate compounds of the formula ##EQU1## where n is any integer greater than, or equal to, two (2) and y is any integer which is equal to, or greater than, zero (0) and less than 3n. Examples are Ferric Sulphate 
     [where y is 0 and n is 2], 
     Fe 5  (SO 4 ) 7  OH [where y is 1, and n is 5] 
     Fe 6  (SO 4 ) 8  (OH) 2  [where y is 2 and n is 6] 
     Fe 7  (SO 4 ) 10  (OH) [where y is 1 and n is 7].

This application is a continuation-in-part of application Ser. No. 07/623,264 filed Dec. 6, 1990 (now abandoned) which is a continuation of application Ser. No. 07/494,808 filed Mar. 14, 1990 (now abandoned) which was a continuation of application Ser. No. 07/281,332 filed Dec. 8, 1988 (now abandoned) which are incorporated herein by reference.

FIELD OF INVENTION

This invention relates to the manufacture of ferric sulphate and other ferric compounds such as hydroxylated ferric sulphate compounds from ferrous sulphate utilizing (NO_(x)) as a catalyst and oxygen as the oxidant. [NO_(x) is known to men skilled in the art as essentially a mixture of NO and NO₂ in undetermined ratios. Thus x is considered to be a number between 1 and 2 inclusive of 1 and 2.]

BACKGROUND OF THE INVENTION

Presently it is known to oxidize FeSO₄ to Fe₂ (SO₄)₃ by HNO₃ (nitric acid) (for example U.S. Pat. No. 2,196,584). The reaction is described by the following simplified equation:

    6FeSO.sub.4 +3H.sub.2 SO.sub.4 +2HNO.sub.3 →3Fe.sub.2 (SO.sub.4).sub.3 +4H.sub.2 O+2NO.

The disadvantage of this process is that the released NO has to be taken out of the reactor, oxidized to NO₂, polymerized to N₂ O₄ and absorbed as HNO₃. The oxidation of NO to NO₂ has to be performed at low temperatures (below 120° C.) and is accompanied by the evolution of a substantial amount of heat, requiring large heat exchangers for the removal of this heat. As the reaction itself is relatively slow to reach completion a large reaction space is needed to provide sufficient residence time and the HNO₃ recovery section adds complications, is bulky and expensive. Another problem is that it is practically impossible to recover 100% of NO_(x) and the emissions are, due to the poisonous nature of both NO and NO₂, environmentally unacceptable.

Another process employed provides for the oxidation of ferrous iron at elevated pH. In this process, it is known that the rate of the oxidation in acidic solutions is negligible for all practical purposes, however the reaction proceeds quite rapidly at pH=5 or higher. This approach has three main disadvantages:

(a) The SO₄ ions coming with the FeSO₄ have to be neutralized.

(b) The products of oxidation (ferric hydroxide and/or oxides) have to be separated from the products of SO₄ neutralization. This operation itself poses a difficult problem which is compounded by the requirements to dispose of the sulphate solution.

(c) Ferric oxides have to be reacted with the full amount of H₂ SO₄ to form Fe₂ (SO₄)₃.

Oxidation in an acidic solution by utilizing a catalyst--charcoal has been performed, however the rate of oxidation increases with molar ratio C/Fe_(t) and at technically feasible values of this ratio the rate of oxidation is quite low. Attempts to increase the rate by increasing the charcoal content introduces frothing problems and a prohibitive cost of charcoal.

Another large group of processes utilizes oxidation by oxygen (either from air or in elemental form) catalyzed by NO_(x) dissolved in the oxidized solution in a form of FeSO₄ *NO.

Typical examples are:

1) United Kingdom Patent No. 17,112, N. McCulloch, 1894,

2) U.S. Pat. No. 4,693,881, R. Miller, 1987,

3) Australian Patent No. 71,741, Y. Mikami, 1974,

4) Japanese Patent No. 49.31638 S. Takada, 1974,

5) Japanese Patent No. 61.286228 Nittetsu Mining KK.

These processes are characterized by low operating temperature (<60° C.) within the range of stability of FeSO₄ *NO complex and the oxygen (or air) is generally bubbled through the reacting solution. Oxygen has to enter solution to react with the NO complex thus the rate of reaction is hindered by low solubility of oxygen and the time required for completion of oxidation is very long. Quoting Mikami: "2.5 1. of ferrous sulphate slurry were oxidized for 17 hours". (This is to compare with our oxidation rate of 2.25 1. completely oxidized in 20 minutes.) Miller in his simplest version releases the desorbed NO_(x) with off-gas to atmosphere. Quoting: "none of the NO which reaches the top of the regeneration reactor is recovered".

The process is not environmentally safe as it discharges large quantities of off-gas containing 0.03-0.15% NO.

Miller tries to solve this problem by complicated systems for recovery of the desorbed NO including conversion to and recovery as HNO₃.

This is to compare with our system where NO is permanently closed in the reactor gas space and only a small bleed-off stream (approximately 3 m³ /h) is, after removal of NO by feed slurry, leaves the process.

It is therefore an object of this invention to provide an improved process for the manufacture of ferric sulphate.

It is a further object of the invention to provide such process which is very efficient and environmentally safe.

Further and other objects of this invention will be realized by those skilled in the art from the following summary of invention and detailed description of the embodiments thereof.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a process for the manufacture of ferric sulphate compounds (including ferric sulphate and hydroxylated ferric sulphate compounds) from ferrous sulphate (in one embodiment crystals, in another embodiment with the addition of pickle liquor) by the oxidation of Fe⁺⁺ →Fe⁺⁺⁺ under pressure utilizing (NO_(x)) as a catalyst and oxygen in a closed circuit or vessel having a liquid (solution) phase and a vapour phase is provided.

According to another aspect of the invention there is provided a process for the manufacture of such ferric sulphate compounds from ferrous sulphate in a closed circuit or vessel comprising a liquid phase and a vapour phase, the process comprising the oxidation between about 70° C. to about 150° C. of Fe⁺⁺ →Fe⁺⁺⁺ under pressure utilizing commercial oxygen in the closed circuit or vessel using (NO_(x)) as a catalyst where x is a number between 1 and 2 inclusive of 1 and 2 and wherein the oxidation of Fe⁺⁺ is affected by spraying the reacting solution containing Fe⁺⁺ introduced to the closed circuit or vessel through a reacting cloud in the vapour phase comprising NO, NO₂ and O₂ enclosed in the vapour phase of the closed circuit or vessel and wherein substantially only the desired end product ##EQU2## is present in the liquid phase of the closed circuit or vessel prior to the addition of FeSO₄ through the reacting cloud in the vapour phase. The process comprises the following reactions ##EQU3## wherein n is any integer greater than, or equal to, two (2) and y is any integer which is equal to, or greater than zero (0) and less than 3n. ##EQU4## is Fe₂ (SO₄)₃ when n is 2 and y is 0. Thus the process comprises the following reactions:

1) NO+1/2O₂ →NO₂, and

2) 2FeSO₄ +H₂ SO₄ +NO₂ →Fe₂ (SO₄)₃ +H₂ O+NO.

Similarly the following ferric sulphate compounds have been produced by the same process steps (with appropriate modifications to yield the desired product);

(a) Fe₅ (SO₄)₇ (OH), where y=1,n=5 ##EQU5## (b) Fe₆ (SO₄)₈ (OH)₂, where y=2, n=6

    [6FeSO.sub.4 +2H.sub.2 SO.sub.4 +3NO.sub.2 ]→F.sub.6 (SO.sub.4).sub.8 (OH).sub.2 +3NO+H.sub.2 O] and

(c) Fe₇ (SO₄)₁₀ (OH), where y=1, n=7 ##EQU6## aspect of the invention there is provided a process for the manufacture of ferric sulphate compounds from ferrous sulphate in a closed vessel containing a liquid phase and a vapour phase, the process comprising the oxidation between about 70° C. to about 150° C. of Fe⁺⁺ to Fe⁺⁺⁺ under pressure utilizing commercial oxygen in the closed vessel using NO_(x) as a catalyst where x is a number between 1 and 2 inclusive of 1 and 2 and wherein the process comprises the following reactions: ##EQU7## wherein n is any integer greater than or equal to two (2) and y is any integer which is equal to, or greater than, zero (0) and less than 3n and wherein the oxidation of Fe⁺⁺ is affected by spraying a solution containing Fe⁺⁺ introduced to the closed vessel through a reacting cloud comprising NO, NO₂ and O₂ enclosed in the vapour space of the closed vessel and wherein the liquid phase fills at least 1/3 of the vessel and substantially only is present in the liquid phase in the closed vessel prior to the addition of FeSO₄ through the reacting cloud in the vapour phase and wherein the liquid phase is also sprayed through the vapour phase.

According the another aspect of the invention there is provided another process for the manufacture of ferric sulphate compounds from ferrous sulphate in a closed vessel containing a liquid phase and a vapour phase, the process comprising the oxidation between about 70° C. to about 150° C. of Fe⁺⁺ to Fe⁺⁺⁺ under pressure utilizing commercial oxygen in the closed vessel using NO_(x) as a catalyst where x is a number between 1 and 2 inclusive of 1 and 2 and wherein the process comprises the following reactions: ##EQU8## wherein n is any integer greater than or equal to two (2) and y is any integer which is equal to, or greater than, zero (0) and less than 3n and wherein the oxidation of Fe⁺⁺ is affected by spraying a solution containing Fe⁺⁺ introduced to the closed vessel through a reacting cloud comprising NO, NO₂ and O₂ enclosed in the vapour space of the closed vessel and wherein the liquid phase fills at least 1/3 of the vessel and substantially only ##EQU9## is present in the liquid phase in the closed vessel during the addition of FeSO₄ through the reacting cloud in the vapour phase and wherein the liquid phase is also sprayed through the vapour phase.

Oxidation of ferrous iron is essentially accomplished in the vapour phase in the reactor which can be a single or multiple vessel/compartment. In the reactor NO_(x) is trapped in the vapor phase (space) above the solution. The present NO then rapidly reacts with the supplied oxygen according to the reaction:

1) NO+1/2O₂ →NO₂.

The solution containing the FeSO₄ introduced to the circuit or vessel is sprayed through this "reacting cloud" (the vapour phase essentially comprising NO, NO₂ and oxygen), cooling the reacting gas by removing the heat of reaction and reacting with formed NO_(x) : ##EQU10## for example 2FeSO₄ +H₂ SO₄ +NO₂ →Fe₂ (SO₄)₃ +H₂ O+NO.

The NO released by reaction 2) is immediately available for reaction 1), NO acts as a catalyst in the oxidation process.

The temperature in the reactor is maintained above the point of decomposition (theoretically 60°-65° C.) of the FeSO₄ *NO complex (which at lower temperatures is stable and is formed at such lower temperatures). Thus NO produced by reaction 2) is released back to the reacting cloud rather than being oxidized in the solution with the consequent low rate of reaction due to the low solubility of oxygen. The oxidation of NO in the gas phase is a relatively fast well-defined reaction. Thus in one application if the reactor is divided into compartments, with FeSO₄ slurry and H₂ SO₄ introduced through the reacting cloud at one end into the first compartment and the reaction is permitted to proceed, with additional product causing overflowing into the next compartment and so on, down the length of the vessel, and oxygen (and HNO₃ make-up for NO_(x)) are added to the last compartment, the NO formed in reaction 1) is converted to NO₂ which will then oxidize another portion of FeSO₄ introduced through the vapour phase.

The produced solution containing traces of NO is denitrated in a NO_(x) desorption vessel by bubbling of feed oxygen through the solution before it is used in the reactor for oxidation. The stream of oxygen then carries the desorbed NO back to the "reacting cloud" in the vapor phase-space of reactor.

The Fe₂ (SO₄)₃ solution for example may then be filtered and stored.

The oxidation may be carried out continuously in different vessels connected one to the other (liquid phase overflowing to next vessel and vapour phases being interconnected and in communication with one another) or may be carried out in a batch in one vessel. The materials may be added in a sluice tank (FeSO₄ and water) and mixed and then fed to continuous process or batch process. The H₂ SO₄ +NO_(x) make-up (HNO₃) are added directly to the reactor.

In the batch process, the oxygen stream from the denitrator is also added to the reactor. When the reactions are completed producing ##EQU11## for example Fe₂ (SO₄)₃, the Fe₂ (SO₄)₃ solution is removed. To produce batches successively in the same vessel, only part (for example 2/3) of the volume is removed, and the remaining portion (for example 1/3) is left. This heats the incoming slurry to the temperature required for initiation of the oxidizing reaction. For the reaction to proceed quickly, the temperature should preferably equal or exceed 90° C. Thus the remaining solution provides the temperature for the initiation of the reaction and the "Heat of Reaction" heats the materials added to the reactor. Additionally the final product ##EQU12## remains in the solution to keep the NO_(x) out of the solution and in the vapour phase (NO_(x) has low solubility in the product ##EQU13## A small portion of gases present in the vapour phase above the slurry solution are bled off to facilitate the removal of any inert gas present and returned to the sluice tank for mixing with the slurry.

The withdrawn ferric sulphate compound solution is denitrated in NO_(x) desorption vessel by bubbling oxygen through the solution and subsequent cooling. The solution may then be filtered and sent to product storage.

The heat recovered from the product cooling heat exchanger may be used for preheating of water or pickle liquor needed for preparation of feed slurry.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be illustrated with reference to processes according to embodiments of the invention illustrated schematically in flowsheet (FIG. 1).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Ferrous sulphate crystals are, after delumping in a suitable crushing device such as a roller mill (position A), transported by screw conveyor (position B) as stream 1 into a sluice tank (position C). In this tank a suitable slurry is formed by an addition of the required amount of water (stream 2). Instead of water a pickle liquor (stream 15) may be used with consequential savings in crystals and sulphuric acid.

A slurry of correct composition (stream 5) for the reaction:

    2FeSO.sub.4 +H.sub.2 SO.sub.4 +1/2O.sub.2 =Fe.sub.2 (SO.sub.4).sub.3 +H.sub.2 O

is fed (stream 7) as well as sulphuric acid (stream 3) and occasionally HNO₃ make-up (stream 4) to a reactor (position E) which can be either continuous-flowsheet (FIG. 1) Details A and B, or batch-Detail C. Detail A illustrates a reactor where a single closed vessel is divided into compartments with the slurry and sulphuric acid added by spraying upstream of the vessel and oxygen and HNO₃ make-up added downstream. The same with Detail B which illustrates three separate vessels E₁, E₂, and E₃ whose vapour phases are connected and whose liquid phases overflow counter-currently to gas phase-slurry added by spraying to upstream vessel E₁ with oxygen downstream at vessel E₃. With respect to Details A, B and C, position G acts as the denitrator.

In the reactor, oxidation under pressure by oxygen (stream 14) takes place, NO_(x) acting as the catalyst of oxidation. The function of NO_(x) can be expressed in a simplified form as:

1) NO+0.5 O₂ =NO₂,

and the ferrous iron is oxidized:

2) 2FeSO₄ +H₂ SO₄ +NO₂ =Fe₂ (SO₄)₃ +NO+H₂ O.

The reaction 1) takes place in the reacting cloud consisting of NO_(x), O₂ and water vapour, located in the vapor space of the reactor. There the produced NO₂ is contacted by spray of the solution and the reaction 2) proceeds.

The product ferric sulphate solution (stream 8), with some NO_(x) dissolved in it due to high partial pressure of NO_(x) in the reactor, is denitrated in the NO_(x) removal vessel (position G), by purging the solution with bubbles of oxygen required for reaction (stream 13). The oxygen with desorbed NO_(x) is then fed to the reactor (stream 14). The NO free solution (stream 9) is cooled in heat exchanger and sent to a pump tank (position K) and pumped (stream 10) through a clarifier filter (position H) to storage (stream 11). Heat removed in the heat exchanger is used for preheating liquids (water or pickle liquor) for slurry preparation. As commercial oxygen always contains some nitrogen, its accumulation in the vapour space of the reactor is avoided by a small bleed-off stream (stream 12). This bleed-off stream is in one application sent to the bottom of the sluice tank (position C) where NO_(x) is removed by contact with FeSO₄ slurry in the sluice tank. This step utilizes the formation of nitrosyl ferrous sulphate in a FeSO₄ solution at temperature <60° C. for the absorption of NO. The NO_(x) catalyst is then returned to the reactor with the feed slurry (stream 7) introduced through the vapour phase.

In other applications the NO containing gas (stream 13) is scrubbed in a ferrous sulphate scrubber (position D) by FeSO₄ slurry (stream 6) because on contact the FeSO₄ dissolves the NO_(x) returning it to sluice tank (position C) and subsequently to reactor (position E). NO_(x) free gas, for additional safety, leaves the process through a caustic scrubber, thus N₂ (nitrogen) and O₂ (oxygen) are permitted to leave to atmosphere.

    ______________________________________                                         1)     Sluice Tank:                                                                   t           ambient,                                                           maximum     60° C.                                               2)     Reactor:                                                                       temperature about decomposition temperature                                                of FeSO.sub.4 *NO complex and                                                  preferably about 70° C. to about                                        150° C.,                                                                most desirable 90°-120° C.                           pressure    20-100 psig,                                                                   preferably 40-60 psig.                                      3)     Product - ferric sulphate solution:                                     typical        190 g. Fe/l,                                                    range          60-250 g. Fe/l.                                                 ______________________________________                                    

With respect to Detail C (which depicts a batch process), when Fe₂ (SO₄)₃ is to be withdrawn, it is preferable that only 2/3 of the volume is removed. The remaining 1/3 volume containing essentially ##EQU14## in this instance Fe₂ (SO₄)₃ is usually at a temperature exceeding 90° C., the temperature at which the reaction proceeds quickly, and thus provides the heat of reaction to cause added cold materials to react quickly to oxidize Fe⁺⁺ →Fe⁺⁺⁺. As well the ##EQU15## in this instance Fe₂ (SO₄)₃ left in the liquid phase keeps NOx in the vapour phase to enable an increased rate of reaction due to the decrease in solubility of NO (product of Fe oxidation) in the Ferric Sulphate. In that case more NO is available for oxidation in the gaseous phase. In the result whereas Mertes (U.S. Pat. No. 2,128,311) whose oxidation was taking place in the liquid phase was achieving oxidation of 0.12 to 0.48 grams of Ferrous to Ferric per liter an hour my process oxidizes in one example, 88 grams Ferrous to Ferric per liter per hour considering reactor space and in the one example 220 grams Ferrous to Ferric per liter per hour considering the liquid phase. Solution accumulates in the reactor. On reaching the high level, the feeding of slurry or solution is stopped and on completion of the oxidation, the product (solution) is released through denitrator (position G) where the product is purged by a stream of oxygen passing from position G into reactor (position E) displacing liquid volume in the reactor (position E).

The following hydroxylated ferric sulphate compounds have been made by carrying out the same process steps. The concentration of the ferrous sulfate and sulfuric acid are changed to produce the proper stoichiometric ratio for each specific compound. The temperature, the pressure and the reaction time are also adjusted within the ranges specified in the patent application.

Fe₅ (SO₄)₇ OH where n=5 and y=1

Fe₆ (SO₄)₈ (OH)₂ where n=6 and y=2 and

Fe₇ (SO₄)₁₀ OH where n=7 and y=1

Thus processes for producing ferric sulphate compounds of the generic formula; ##EQU16## have been described by oxidation of ferrous sulphate in accordance with the equation: ##EQU17##

As many changes can be made to the embodiments of the invention without departing from the scope of the invention, it is intended that all material contained herein by interpreted as illustrative of the invention and not in a limiting sense. 

The embodiments of the invention in which an exclusive property or privilege is claimed are as follows:
 1. A process for the manufacture of ferric sulphate compounds from ferrous sulphate in a closed vessel containing a liquid phase and a vapour phase, the process comprising the oxidation between about 70° C. to about 150° C. of Fe⁺⁺ to Fe⁺⁺⁺ under pressure utilizing commercial oxygen in the closed vessel using NOx as a catalyst where x is a number between 1 and 2 inclusive of 1 and 2 and wherein the process comprises the following reactions: ##EQU18## wherein n is any integer greater than or equal to two (2) and y is any integer which is equal to, or greater than zero (0) and less than 3n and wherein the oxidation of Fe⁺⁺ is affected by spraying a solution containing Fe⁺⁺ introduced to the closed vessel through a reacting cloud comprising NO, NO₂ and O₂ enclosed in the vapour space of the closed vessel and wherein the liquid phase fills at least 1/3 of the vessel and substantially only ##EQU19## is present in the liquid phase in the closed vessel prior to the addition of FeSO₄ through the reacting cloud in the vapour phase and wherein the liquid phase is also sprayed through the vapour phase.
 2. The process of claim 1 wherein y is not zero.
 3. The process of claim 1 wherein the vessel in which reaction 1) takes place is divided into compartments with FeSO₄ solution or slurry introduced at one end into the first compartment and the reaction is permitted to proceed, with additional solution when sprayed through the reacting cloud and added to the solution in the vessel or circuit causing overflowing into the next compartment and so on, down the length of the vessel, and oxygen is bubbled through the liquid phase of the last compartment to serve as a denitrator, to remove NOx and proceeds with the removed NO_(x) counter-currently to the stream of the solution, the NO formed by reaction 1) being converted in the vapour phase of each compartment to NO₂ which then oxidizes the ferrous sulphate sprayed through the reacting cloud of the compartment or proceed upstream through the vapour phases of the previous compartments of the vessel to react with ferrous sulphate sprayed through the vapour phase in the previous compartments.
 4. The process of claim 3 wherein y is not zero.
 5. The process of claim 1 wherein the process is carried out continuously in one vessel.
 6. The process of claim 5 wherein y is not zero.
 7. The process of claim 5 wherein the FeSO₄ and water are added in a sluice tank and mixed and the resultant solution is added to the vessel.
 8. The process of claim 5 wherein the ##EQU20## solution is supplied to a denitrator where oxygen is supplied to denitrate said solution and the resultant gas is then passed to said vessel from the denitrator.
 9. The process of claim 8 wherein the ##EQU21## solution is removed through the denitrator.
 10. The process of claim 1 wherein successive batches are produced in the same vessel, and part of the volume of the earlier batch is removed leaving a portion comprising substantially only ##EQU22##
 11. The process of claim 1 wherein Fe₅ (SO₄)₇ (OH) is produced.
 12. The process of claim 1 wherein Fe₆ (SO₄)₈ (OH)₂ is produced.
 13. The process of claim 1 wherein Fe₇ (SO₄)₁₀ (OH) is produced.
 14. A process for the manufacture of ferric sulphate compounds from ferrous sulphate in a closed vessel containing a liquid phase and a vapour phase, the process comprising the oxidation between about 70° C. to about 150° C. of Fe⁺⁺ to Fe⁺⁺⁺ under pressure utilizing commercial oxygen in the closed vessel using NOx as a catalyst where x is a number between 1 and 2 inclusive of 1 and 2 and wherein the process comprises the following reactions: ##EQU23## wherein n is any integer greater than or equal to two (2) and y is any integer which is equal to, or greater-than zero (0) and less than 3n and wherein the oxidation of Fe⁺⁺ is affected by spraying a solution containing Fe⁺⁺ introduced to the closed vessel through a reacting cloud comprising NO, NO₂ and O₂ enclosed in the vapour space of the closed vessel and wherein the liquid phase fills at least 1/3 of the vessel and substantially only ##EQU24## is present in the liquid phase in the closed vessel during the addition of FeSO₄ through the reacting cloud in the vapour phase and wherein the liquid phase is also sprayed through the vapour phase.
 15. The process of claim 14 wherein y is not zero.
 16. The process of claim 14 wherein the vessel in which reaction 1) takes place is divided into compartments with FeSO₄ solution or slurry introduced at one end into the first compartment and the reaction is permitted to proceed, with additional solution when sprayed through the reacting cloud and added to the solution in the vessel or circuit causing overflowing into the next compartment and so on, down the length of the vessel, and oxygen is bubbled through the liquid phase of the last compartment to serve as a denitrator, to remove NOx and proceeds with the removed NO_(x) counter-currently to the stream of the solution, the NO formed by reaction 1) being converted in the vapour phase of each compartment to NO₂ which then oxidizes the ferrous sulphate sprayed through the reacting cloud of the compartment or proceed upstream through the vapour phases of the previous compartments of the vessel to react with ferrous sulphate sprayed through the vapour phase in the previous compartments.
 17. The process of claim 14 wherein the process is carried out continuously in one vessel.
 18. The process of claim 17 wherein the FeSO₄ and water are added in a sluice tank and mixed and the resultant solution is added to the vessel.
 19. The process of claim 17 wherein the ##EQU25## solution is supplied to a denitrator where oxygen is supplied to denitrate said solution and the resultant gas is then passed to said vessel from the denitrator.
 20. The process of claim 19 wherein the ##EQU26## solution is removed through the denitrator.
 21. The process of claim 14 wherein successive batches are produced in the same vessel, and part of the volume of the earlier batch is removed leaving a portion comprising substantially only ##EQU27## 